An electric power supply system for supplying electric power from an electric power supply to an electric instrument is installed in vehicles. In this electric power supply system, if, due to a short circuit, an overcurrent flows through a conductive wire that connects the electric power supply and the electric instrument, there is a risk that the conductive wire ignites. Accordingly, the electric power supply system is provided with a preventive apparatus (see JP3589392, for example) for preventing an overcurrent from flowing through the electric current path from the electric power supply to the electric instruments.
In the preventive apparatus disclosed in JP3589392, the electric power is supplied to a load from an electric power supply via FETs (Field Effect Transistors). If a current of at least a predetermined magnitude flows through an electric current path from the electric power supply to the load, the FETs interrupt the current flowing through the electric current path.
Conventional preventive apparatuses include a preventive apparatus that is provided with a switch on an electric current path from an electric power supply to a load, and compares an electric potential at one end, on the load side, of the switch with a threshold electric potential. If the switch is, for example, a semiconductor switch, and a large current flows through the electric current path, then the electric potential at the one end, on the load side, of the switch will significantly decrease due to a voltage drop caused by the on-resistance of the switch.
In the above-described preventive apparatus, if the electric potential at the one end, on the load side, of the switch is reduced to a value that is lower than the threshold electric potential, then the switch will be turned off. Accordingly, an overcurrent is prevented from flowing through the electric current path.
However, if the load is inductive, an inrush current will flow through the electric current path immediately after the load is started. In the above-described preventive apparatus, the switch is at risk of being turned off by mistake when an inrush current flows through the electric current path.
Furthermore, the temperature in the electric power supply system changes depending on the environment in which the vehicle is running, the operational state of the load, or the like. The value of the current that is allowed to flow through the switch varies according to the temperature in the region of the switch. If the temperature in the region of the switch is high, the switch may fail even if the value of the current flowing through the switch is relative small. Therefore, in the above-described preventive apparatus, if the threshold electric potential is set to a constant value, there will be a risk that an overcurrent whose value exceeds the value of the current that is allowed to flow through the switch flows through the electric current path.
Furthermore, in a case of mass production of the above-described preventive apparatuses, there is a large variation in the on-resistance values of the switches. Accordingly, assuming that the same threshold electric potential is set for all the preventive apparatuses, there is a risk that, in the preventive apparatus that is provided with a switch having a small on-resistance value, an overcurrent flows through the electric current path even if the electric potential at one end, on the load side of the switch is higher than the threshold electric potential.
The present invention was made in view of the above-described circumstances, and it is an object thereof to provide a preventive apparatus that can reliably prevent an overcurrent from flowing through an electric current path.